1. Field of the Invention
The present invention generally relates to a correction circuit which controls parameters of a circuit to be corrected by utilizing an oscillation frequency of an oscillator circuit.
A voltage-controlled oscillator (VCO) is applied to various electronic devices such as portable radio communication devices and modems. Examples of portable devices are telephone sets, mobile telephone sets and cordless telephone sets. More particularly, the VCO is used for, for instance, frequency tuning, signal detection, data reproduction and clock reproduction. In many cases, the VCO is used together with a filter. It is required that the VCO and filter have a high precision level. The VCO which does not have a high precision will not oscillate an expected frequency even if a designed voltage is applied thereto. If the filter does not have a high precision, the selectivity thereof will be degraded.
Normally, the VCO and filter include a resistor and a capacitor. Hence, the precision of the VCO and filter, in other words, the characteristic thereof depends on the absolute precision of the circuit components (elements) such as a resistor and capacitor. Generally, the higher the precision, the more expensive the circuit components. Hence, use of highly precise circuit components is not a practical solution to obtain a given characteristic.
The present invention is directed to providing a correction circuit which makes it possible to realize a given circuit characteristic even if the circuit components have different precision levels.
2. Description of the Related Art
FIG. 1A is a circuit diagram of a conventional VCO. An input voltage Vin is converted into a current I (=Vin/R) by means of a voltage/current converter circuit made up of an operational amplifier 10, a P-channel MOS transistor 11 and a resistor R. A voltage equal to the gate voltage of the transistor 11 is applied to the gates of P-channel MOS transistors 12 and 14, and is thus ON. The above gate voltage is the output voltage of the operational amplifier 10. N-channel MOS transistors 13 and 15 form a current-mirror circuit. Thus, if the transistors 13 and 15 have an identical size, the same amount of currents as that of the current flowing in the transistor 11 will flow in the transistors 13 and 15.
A node n1 is connected to a capacitor C, the inverting input terminal of an operational amplifier 16, and the non-inverting input terminal of an operational amplifier 17. At the node n1, the drains of the transistors 14 and 15 are connected together. The non-inverting input terminal of the operational amplifier 16 is supplied with a high-level reference voltage Vrh. The inverting input terminal of the operational amplifier 17 is supplied with a low-level reference voltage Vrl. The operational amplifiers 16 and 17 respectively function as comparators. The output signals of the operational amplifiers 16 and 17 are latched in a latch circuit 18. One of the latched output signals of the operational amplifiers 16 and 17 is externally output as an oscillation output signal Vout of the VCO, and the other latched output signal is sued to control the ON/OFF of switches SW1 and SW2.
FIG. 1B is a waveform diagram showing an operation of the VCO shown in FIG. 1A. When the voltage Va of the node n1 becomes higher than the low-level reference voltage Vrl, the latch circuit 18 outputs the high-level signal, which is applied to the switch SW1 and is applied to the switch SW2 via an inverter 20. Hence, the switch SW1 is closed and the switch SW2 is opened. Thus, the capacitor C connected between the node n1 and the ground is charged through the transistor 14. When the voltage Va reaches the high-level reference voltage Vrh, the latch circuit 18 outputs a low-level output signal, by which signal the switch SW1 is opened and the switch SW2 is closed. Thus, the charge stored in the capacitor C flows to the ground through the transistor 15. That is, the capacitor C is discharged.
The oscillation cycle T of the above VCO is expressed as follows: EQU T.alpha.[(Vrh-Vr1)RC]/Vin.
It can be seen from the above that the oscillation cycle T is proportional to the product of the resistance R and the capacitance C. In other words, the oscillation frequency of the VCO is inversely proportional to the product of the resistance R and the capacitance C.
FIG. 2 is a circuit diagram of a conventional filter circuit, which is made up of an operational amplifier AMP, resistors R and R1' and a capacitor C. The cutoff frequency fc of the filter circuit shown in FIG. 2 is described as follows: EQU fc=1/(2.pi.RC).
It can be seen from the above that the cutoff frequency fc is proportional to the reciprocal of the product of the resistance R and the capacitance C.
The VCO shown in FIG. 1A and the filter shown in FIG. 2 have respective characteristics (that is, the oscillation frequency and cutoff frequency) which depend on the precision levels of the resistor R and the capacitor C, because the characteristics depend on the product of the resistance R and the capacitance C. If the resistor R and the capacitor C are realized on a chip by the integrated circuit technology, the resistors R and the capacitors C formed on the respective chips will have different resistance values and different capacitance values. Such a desperation results from an error introduced during the production process. For example, the resistor is realized by implanting ions in a substrate, and the capacitor is realized by using an oxide film. In this case, an error which occurs in the ion implantation, the pattern size, the thickness of the oxide film and so on affects the characteristics of the circuit elements. Hence, the VCO and filter circuits including these circuit elements formed on the chips will have different characteristics. In this case, it is required to select chips which satisfy required characteristics. However, this selecting work is cumbersome in practical use.